This invention relates to a technique for the deactivation of residual sodium metal in general and in Liquid Metal Fast Breeder Reactor (LMFBR) systems in particular. Sodium metal is most commonly used in LMFBR systems as a heat transfer fluid because of its good technical characteristics, including a low melting point (370.7 K), high boiling point (1153 K), a water-like viscosity when molten (0.4519 cP at 473 K), a high heat transfer coefficient (0.820 W/cm-K at 473 K), and a high thermal heat capacity (1.34 J/g-K) at 473 K). Also, it is less chemically hazardous than other metals such as sodium-potassium alloy (NaK), and so allows for easier system maintenance activities. Residual sodium is defined as the sodium metal remaining behind once the bulk sodium has been drained from any commercial application but specifically from these systems for reasons of maintenance or decommissioning work. The residual sodium may either be in the form of a thin coating on vertical surfaces or thicker pools at the bottom of vessels, elbows, horizontal runs of pipe, etc. This residual sodium must also be removed in order to make the system safe for exposure to open air and often to comply with regulations that require the removal of the hazardous components in order to begin decommissioning activities. Although the specific examples hereinafter set forth refer to LMFBR systems, the invention is far broader and includes treatment of any residual sodium in any commercial or industrial application, and is not limited to nuclear reactors.
Currently, the best-established method for removing residual sodium is chemical deactivation of the residual sodium using steam and nitrogen or water-saturated nitrogen, followed by a liquid water wash to remove the reaction products. The steam or water vapor is used to convert the residual sodium into sodium hydroxide and hydrogen, while the nitrogen is used to dilute the hydrogen and to prevent the intrusion of air into the system being treated. Once the sodium metal has been converted into sodium hydroxide, the sodium hydroxide can be safely water-flushed without the hazard of generating an explosive hydrogen atmosphere.
The technique of using steam and nitrogen is commonly used in industry to remove residual sodium. For instance, Safety-Kleen, Inc., one of the United State's largest environmental services companies, routinely uses steam and nitrogen to clean residual sodium from storage tanks and other systems, including two used sodium tanks that were located at Argonne National Laboratory-West. In addition, E. I. DuPont de Nemours, Inc., also uses this technique to clean sodium rail cars once they have been drained of bulk sodium.
Steam and nitrogen have been used to clean nuclear systems containing residual sodium. In 1968, Atomics International, a Rockwell International company, deactivated the residual sodium in the primary heat transfer system at the Hallam Nuclear Power Facility, which was located south of Lincoln, Nebr., using a steam-nitrogen mixture. Argonne National Laboratory commonly uses the technique to remove residual sodium, though not in-situ, from smaller parts that are either coated with sodium or contain residual amounts of sodium, such as valves or small containers that were used in nuclear applications.
A modification of the steam-nitrogen technique was developed by Merrick Remediation Company, Inc., another environmental services company located in the United States. The technique, designed mainly for treating residual sodium in large vessels, uses water-saturated nitrogen gas to react residual sodium.2,3 Water-saturated nitrogen is forced by pressure differential into the vessel being treated. The vessel is maintained at a lower temperature than the incoming gas stream so that liquid water condenses from the vapor stream and reacts with the residual sodium. The sodium hydroxide created by the water-sodium reaction is removed as an aqueous solution by suction, and the hydrogen gas is vented as a mixture of nitrogen and hydrogen. Using humidified nitrogen instead of steam and nitrogen eliminates the need for a steam source at the treatment site. Also, it reduces the hazard to workers by reducing the surface temperatures of process piping.
While these methods are effective at deactivating residual sodium, there are two disadvantages inherent in either technique. The first disadvantage is in the lack of control of the reaction process. On a crude level, the deactivation reaction is controllable in that turning on the flow of steam or water-saturated nitrogen can start the reaction, and stopping the flow of steam or water-saturated nitrogen can stop the reaction. The reaction temperatures and pressures, however, can be unstable, especially if the layer thickness of sodium hydroxide solution is allowed to increase beyond approximately one centimeter on top of the residual sodium. According to an unpublished report by Atomics International, the company that performed the deactivation of residual sodium in the Hallam Reactor, a concentration gradient is established in the liquid layer between the surface of the residual sodium and the surface of the sodium hydroxide layer with the lowest concentration of sodium hydroxide (and the highest water concentration) occurring at the exposed sodium hydroxide surface. Occasionally, the movement of hydrogen bubbles or other mechanical disturbances can cause the layer to circulate, leading to the sudden exposure of residual sodium to a solution that is rich in water. This can cause sudden spurts of chemical reactivity, which can lead to sudden surges in temperature, pressure, and in hydrogen production.
We have measured direct evidence of this instability. Inside an instrumented test chamber, 0.025 kg of sodium metal was exposed to a mixture of steam and nitrogen flowing at a rate of 0.45 kg/hour and 0.70 kg/hour, respectively. The temperature of the incoming gas mixture was 356 K. The temperature of the sodium sample was measured along with the chamber pressure and the concentration of hydrogen in the off-gas. FIG. 1 shows that the temperature of the specimen varied widely between 356 K and as high as 708 K. The temperature spikes were accompanied by surges in system pressure and hydrogen concentration. Larger surges in pressure may have occurred during the test, but the pressure instrumentation and control equipment limited the recorded readings to no higher than +20.7 kPa (3 psi).
The second disadvantage is the creation of concentrated solutions of sodium hydroxide. Concentrated sodium hydroxide solutions are corrosive to equipment and hazardous to workers. Often sodium hydroxide solutions must be neutralized before disposal, which adds to the expense of the deactivation process and creates a larger volume of waste.
As an alternative to the established residual sodium deactivation techniques, we offer a unique process for deactivating residual sodium that does not suffer from temperature and pressure instabilities and does not produce waste that requires neutralization. Unlike conventional deactivation techniques that use steam-and-nitrogen or water-saturated nitrogen to convert residual sodium metal into sodium hydroxide, this process uses humidified (but not saturated) carbon dioxide at ambient temperature and pressure to convert residual sodium into solid sodium bicarbonate.